fix(storage): purge orphaned feature_set_members after the refactor (migration 017)

[its-mash]

Symptom

After upgrading stable → beta, the Starter FeatureSet shows a member count (e.g. "93 members") but the detail panel shows 0 selected, and it persists even after uninstalling every server — i.e. the set effectively grants 0 tools despite the count.

Root cause

The #151 refactor changed a feature member's identity from a qualified server_id/tool_name string to the server_features.id UUID, but no migration converted the rows that migration 001's default (now "Starter") set had accumulated under the old model. Those members survived as orphans whose member_id resolves to no live feature:

• the FeatureSets card counts the raw feature_set_members rows → "93",
• the detail/resolver match member_id against live server_features.id → 0.

Migration 006 collapsed the all/server-all sets and deleted their members, but never reconciled the surviving default set's members.

Fix — migration 017

Delete feature_set_members that point at a feature or feature set that no longer exists:

DELETE FROM feature_set_members WHERE member_type='feature'
  AND member_id NOT IN (SELECT id FROM server_features);
DELETE FROM feature_set_members WHERE member_type='feature_set'
  AND member_id NOT IN (SELECT id FROM feature_sets);

These members already resolve to nothing, so this changes no effective behavior — it makes the count honest and leaves Starter empty, identical to a freshly created Space. (Confirmed earlier: the resolver never falls back to the Starter/active set — unmapped sessions deny and expose only @mux meta-tools — so an empty Starter is correct, not a regression.) Idempotent; any valid member a user added under the new model is kept.

Tests

tests/database/migrations.rs::test_017_purges_orphaned_feature_set_members seeds one valid + two orphaned members and asserts only the resolvable one survives. All 5 migration tests pass; pre-commit (fmt/clippy/eslint/typecheck) green.

Follow-up (not in this PR)

Uninstalling a server deletes its server_features but not the feature_set_members referencing them, so a custom set can re-accumulate orphans over time. Options: cascade the cleanup in delete_by_server, or have the FeatureSets card count only resolvable members. Happy to do either next.